The E-value is a measure of the energy performance of a building and its fixed installations in standard conditions. In Belgium the system called EPB (energy performance of buildings) is used to study the energy performance of buildings. The maximum E-value in Flanders, Belgium currently is E50 but for the year 2020 is set for E35. The main factors that influence the E-value are: the isolation grade of the building, air tightness of a building, compactness, orientation and overheating. The isolation grade is in the centre of the U-value of the walls, roof and windows combined with the thermal bridges. Isolation and minimizing thermal bridges are key aspects to reduce heat loss resulting in more energy efficiency. Next to these key aspects the following topics also play a role in the E-value. The airtightness of a building is its ability to keep air currents from the inside in and vice versa. Currently there is no set limit in Belgium but the aim is to keep it as low as possible. The compactness of a building is defined as the ratio of the protected volume over the total loss of the surface area, this comes down to the more compact the building the more energy efficient it is. The orientation plays a key role in optimizing heat gain by the sun. A good orientation for a passive house can make a 60-70% positive heat gain difference and for a regular home 5-20%. This makes a big difference in energy costs. This is gained by having a low g-factor in the windows. When talking about installations the following subject are included: heating systems, ventilation systems, Renewable systems and lights (only in industrial buildings). Overheating is a subject which is negative for the E-value because it works against all the factors that improve the E-value. Because when overheating occurs you want to get rid of the heat inside a building which is the exact opposite of the previously mentioned importance of isolation.